Landscaper’s Perspective on Plant Diseases | Pests

Plant diseases | Plant pests | Plant weeds | Lawn diseases | Lawn pests | Lawn weeds

The complex environment of living plants and other organisms present in your landscape provide the “home” for a wide variety of diseases and insects. Some of these “pests” can prove troublesome if not treated or removed. The following list of common problems should help you recognize and take steps to combat the “critters”.

Plant diseases

“Shot Hole” fungus

Fruiting and ornamental varieties of plum and cherry trees are susceptible to this disease, which creates small holes in the leaves, sometimes in large quantities. Shothole is very common especially in very wet areas. It is generally not fatal but it is unsightly. Fungicide applications during the growing season provide control.


A powdery white substance present on the leaves and buds of plants like roses, lilacs, and exbury azaleas indicates mildew was developed during warm, humid conditions. Leaves of the affected plants tend to curl up. Fungicide applied during the growing season will help control the disease.


This is a common problem with Dogwood trees. Brown blotches along the mid-vein of the leaf are indicative of the disease. Especially damaging after a very wet spring, leaves often fall off and the disease can move into the branches. Pre-bud break fungicide applications are recommended along with removal of affected branches.

Verticillium wilt

This is an incurable disease that can slowly kill ornamental trees like the very popular Japanese maple varieties. A branch will suddenly die on infected trees and others will slowly follow. The soil around the tree should be considered contaminated and trees of the same variety or specie should not be replanted.


Roses and Photinia are common hosts for this disease. Leaves turn yellow or “black” and fall off. A fungicide on the affected plants will help control this disease.

Plant pests


All the lifecycle stages of this pest can damage plant material. These black or brown adult insects, about 1/4″ long, chew the edges of leaves during the summer months. This “notching” is not fatal but should alert you to the fact that the adults will soon lay eggs. The larva, or grubs, will cause severe damage if left untreated. During the spring the grubs can “girdle” the trunk of a plant and/or destroy the root zone very quickly. These pests move from plant to plant rapidly so treatment(s) with an insecticide is important.

Leaf miner

Birch trees are a common host for this small caterpillar. Leaves may roll up or have patches missing as this insect chews it’s way through your trees. Several applications of insecticide are required to rid your trees of this pest.


These reddish brown mites seem to prefer tightly branched evergreen plants. If you notice yellowing of the foliage, the spidermite may be present. To check for the mite, hold a white piece of paper under a branch or branches, shake the branches over the paper and then rub or smear your hand over the paper. If reddish streaks appear on the paper, you have mites! Systemic insecticides work well on this critter.


The telltale “slimy” trail of this nocturnal mollusk probably means a feeding frenzy on your favorite plants. As evidenced by holes in the foliage, slugs can be very destructive to succulent plants. Slug bait encircling the plant can stop them in their tracks.


These small soft-bodied insects of various colors, can suck the life out of your plants. A shiny, sticky secretion on the foliage can indicate a problem, especially on new growth. Give them a blast of water to remove them from the plant or repeated applications with an insecticide may be needed for longer control.

Tent Caterpillars

This pest is very obvious from the unsightly “tent” formed high up in the host tree or shrub. Seldom fatal, the most efficient control is removal by cutting out the tent.

Lace Bugs

Lace Bugs are a newcomer to the Northwest.
They feast on azaleas and rhododendrons, turning the leaves a pale yellow color. Although not typically fatal to the plants, effective systemic or organic treatments are available.

Not a disease or pest – it’s sunscald

This is often mistaken for disease or bug problems, but browned curled leaf margins may be the result of intense heat. Make sure that the plant is getting enough water and remove the affected leaves to allow new growth to generate.

Plant weeds

A wide variety of “broadleaf” and “grassy” weeds can germinate in newly tilled soil. “Contact” weed killers, like “round-up” can be useful in controlling most weeds already present in the shrub beds. Be careful not to get the spray on the foliage of your plants! As a “pre-emergent” control, “Dimension”and “Rhonstar” are common granular products for weed control applied in February and November.

Lawn diseases


The most common lawn disease, it appears as yellow patches turning brown that are held together by red or pinkish webbing on the grass blades. It is usually a symptom of low fertility, which requires treatment with a high nitrogen fertilizer. Maintaining a consistent fertilization program will help prevent recurrence.

Lawn pests


The larval stage of this insect appears in the spring with an appetite! Patches of grass will die out or look faded as the roots are devoured by the chewing grubs. The 1/2″ long grubs are brownish with black heads. Usually spring and fall applications of insecticides are recommended. The adult cranefly has wings and looks like a giant mosquito.


These voracious mammals can destroy your lawn or shrub beds in a short period of time. Moles are extremely difficult to eradicate. Trapping may be the most successful, yet frustrating method to rid your yard of the critters causing the dirt piles and soft “tunnels”. Perhaps a professional service is in order for a significant population.

Lawn weeds

The most common weeds are broadleaf varieties like dandelion, clover, spurge, and plantain. They are best controlled with contact herbicides available at most garden shops. Read the directions carefully for the correct mixture. Sometimes “grassy” weeds invade a stand of grass. Lighter color or different texture grass must be killed first and the area re-seeded or re-sodded. Unfortunately, in our climate, the invasive grasses commonly germinate and contaminate our otherwise uniform lawns.

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